The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the production of a black blood magnetic resonance angiogram ("MRA") from a fast spin echo scan.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the xy plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There are a number of fast pulse sequences available on commercial NMR scanners which enable fast scans to be performed. One of these fast pulse sequences is the fast-spin-echo ("FSE") pulse sequence, also known as the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled "RARE Imaging: A Fast Imaging Method for Clinical MR." The FSE sequence utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill sequence to produce multiple spin echo signals from a single excitation pulse. Each pulse sequence, or "shot," results in the acquisition of a plurality of views, and a plurality of shots are typically employed to acquire a complete set of image data. For example, an FSE pulse sequence might acquire 8 separate echo signals, per shot, and an image requiring 256 views would, therefore, require 32 shots.
In clinical scans using FSE pulse sequences, more than one image is often acquired during a single scan. The images depict the same structures in the patient, but different tissues are enhanced in each image. The tissue contrast in each image is affected by intrinsic NMR parameters, such as proton density and T.sub.1 and T.sub.2 values. Those tissues having a higher proton density or shorter T.sub.1 value will be bright in the first-echo image. Such tissues include, for example, fat, gray matter and white matter. On the other hand, those tissues such as cerebrospinal fluid, edema and tumor that have longer T.sub.2 values , but not very long T.sub.1 values as compared to the repetition time TR, will be bright in the second-echo image. Such multi-image fast spin echo imaging sequences are described by N. Higuchi et al in an abstract in Journal of Magnetic Resonance Imaging, Vol. 1, No. 2, pg. 147, 1991 entitled "Two-Contrast RARE: A Fast Spin-Density and T.sub.2 -Weighted Imaging Method."
Magnetic resonance angiograms (MRA) are produced in scans which employ specialized NMR pulse sequences. There are two basic methods used in MR angiography. In one method, know as the "time-of-flight" or "inflow enhancement" method, enhancement occurs when unsaturated spins flow into a slice which has been excited by many radio-frequency (RF) pulses. If the time between RF pulses is much shorter than the T.sub.1 relaxation rate of the tissues, the longitudinal magnetization does not have time to recover before the next RF pulse is applied. This results in reduced transverse magnetization and reduced signal when the magnetization is again tipped into the transverse plane by the next RF excitation pulse. The inflowing blood, on the other hand, will have seen no prior RF pulses and will therefore have a large longitudinal component of magnetization, which produces a larger transverse magnetization and a larger NMR signal. As a result, the flowing blood appears brighter in the reconstructed image. The second method, known as the "phase contrast" method, acquires two images in which the phase of the signals produced by moving blood is different. A bright image of the vascular structure is produced using the phase difference information in the two images. Such MRA scans employ separate programs which are dedicated solely to this purpose.
A variant of the conventional MR angiogram is the "black blood" angiogram in which flowing blood appears darker than surrounding stationary tissues. This method is described, for example, by Robert R. Edelman et al in "Extracranial Carotid Arteries: Evaluation With `Black Blood` MR Angiography", Radiology, 1990; 177:45-50, by Daisy Chien et al in " Turbofisp Black Blood Angiography", SMRM 1991; and by Daisy Chien et al in " High Speed Black blood Imaging of Stenosis in the Presence of Pulsatile Flow", SMRM 1992. Black blood angiograms have significant clinical use, but they are produced with special purpose NMR scans using specially designed pulse sequences.